Various types of apparatus require limit switches in order to stop a moving element driven by a motor when a predetermined deflection or end position has been reached. Such limit switches are usually constructed in the form of microswitches or the like and interrupt power to a motive source, typically interrupting electrical power to an electric motor. If rotating elements are used, sliding contacts may be used as limit switches which, when a particular point is exceeded, interrupt current supply to the motor.
The limit switches have the disadvantage that, to reverse rotation of the motor, complex circuits are needed in order to permit re-energization of the motor in reverse direction after the limit switch has been operated, requiring a plurality of microswitches; if sliding contacts are used, the reliability of contact engagement is impaired due to dirt and other possible interruptions of the circuit and, additionally, construction of such contacts for reliable operation is comparatively expensive.